1. Field of the Invention
The present invention is concerned generally with silver compositions of increased hardness and is particularly directed to silver alloys containing intermetallic compounds which can be subsequently heat treated to provide exceptional and reversible hardness.
2. Description of the Prior Art
It has been recognized for many centuries that pure silver is extremely soft and must be strengthened for even the least demanding applications. For this reason, many methods of hardening silver have been introduced: the primary methods being alloying and mechanical working. Mechanical working will increase the disorder in the silver metal crystal producing a phenomenon known as work hardening. This process is reversible in that elevated temperatures return the strength of the metal to that of the unworked solid solution or the pure metal. Unfortunately, undesireable hardening often takes place during the forming of silver articles. The metal becomes harder as it is formed, not softer, and subsequent application of heat to the fully formed article part will soften it. Although, many metalsmiths continue to take advantage of the added strength obtained by mechanical working of the metal, this method of hardening cannot always be employed and does not always permit the optimum hardness during processing.
In comparison, the method of alloying achieves added strength through solid solution hardening. It is commonly recognized that a mixture of two different metals is always stronger than one of the two pure metals itself. The traditional alloy of pure silver is sterling silver consisting of 92.5% by weight of pure silver and 7.5% by weight of copper. This form of hardening is not reversible in that the alloy once formed cannot be returned to the strength of the individual metals that formed it. It is generally necessary to work alloys at their full strength.
While other methods of strengthening precious metals are known including control of the grain size and crystal dispersion strengthening, the magnitude of the strengthening is found to be very small at best. Other methods of hardening such as ordered solution hardening or phase transformation hardening, while effective, are not known for use in silver or silver alloys. As a result, the only practical approach has become the preparation of different silver containing alloys which are then mechanically work hardened or age hardened at elevated temperatures to provide sterling silver alloys of increased hardness.
Representative of this general approach and of the developments in recent years are the following patents: U.S. Pat. No. 1,022,600 describing a silver alloy composed principally of silver, copper, and traces of titanium; U.S. Pat. No. 1,928,429 describing an annealed alloy consisting of silver from about 50-90%, beryllium from about 0.10-2.5%, and copper; U.S. Pat. No. 1,970,319 describing a tarnish-resisting silver alloy made from about 85-93% silver, tin, and up to 4% of either cadmium, antimony, copper, zinc, manganese and nickel-chromium; U.S. Pat. No. 1,984,225 describing an age hardening process for hardening silver and a silver alloy containing at least 92.5% silver, aluminum, and copper; U.S. Pat. No. 2,196,302 describing a silver alloy containing silver, copper, and lithium; U.S. Pat. No. 2,196,303 which describes another alloy containing silver, lithium, and copper in varying proportions; U.S. Pat. No. 2,235,634 which describes a silver solder whose essential ingredients are silver, copper, and lithium; and British Pat. No. 573,661 which describes a silver solder alloy consisting of silver, copper, tin, and zinc.
Despite these innovations and the introduction of the age hardening process to increase the hardness of silver and silver alloys, the presently available sterling silver alloys are relatively soft. For this reason, a sterling silver alloy which could be subsequently hardened and which would then demonstrate significant increases of hardness as well as reversible hardness would represent a major advance and improvement in this art. Insofar as is presently known, sterling silver alloys demonstrating exceptional and reversible hardness, though highly useful and desireable, have not been available.